Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies by oxidation of a fuel and reduction of an oxidizing agent. The fuel and the oxidizing agent, undergo a redox reaction at two isolated electrodes, each containing a catalyst in contact with an electrolyte. The electrolyte is located between the electrodes to prevent direct reaction of the two reactants and to conduct ions from one side of the cell to the other. Advantageously the electrolyte can be a solid polymer electrolyte.
A broad range of reactants can be used in fuel cells. For example, the fuel may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
On the other hand, an electrolyzer uses electricity to produce different chemical species, e.g. hydrogen and oxygen from water or chlorine, sodium hydroxide and hydrogen from an alkaline brine. An electrolyzer basically involves a fuel cell operating in reverse.
Fuel cells that allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input, are generally referred to as “reversible” or “regenerative” fuel cells. The ability to generate electricity and regenerate fuel makes reversible fuel cells particularly attractive for electrical energy storage.
Of particular interest are the so-called unitized regenerative fuel cells, that is reversible fuel cells wherein both the energy generation mode of operation (the fuel cell mode) and the electrolysis mode of operation are carried out within the same cell stack. Because the electroactive components of such a cell must operate in both electrolysis and fuel cell modes, it is difficult to optimize them for both.
As an example U.S. 2003/0068544 A (CISAR, A.) Oct. 4, 2003 discloses an unitized regenerative hydrogen-oxygen fuel cell wherein the oxygen electrode comprises an electrocatalyst layer containing a mixture of a catalyst active for the evolution of oxygen from water and of a catalyst active for the reduction of oxygen to water. The gas diffusion layer for the oxygen electrode comprises hydrophobic and hydrophilic regions. With such a configuration however the transport of water to and from the differently active electrocatalytic areas of the electrode does not appear to be optimal.
Thus, the need still exists for unitized regenerative fuel cells capable to operate with the highest efficiency both in the fuel cell mode and in the electrolysis mode.